Treat Balb/c and Balb/cfC3H mice in vivo with chemical carcinogens and monitor for tumor incidence and expression of MuMTV via nucleic acid hybridization techniques, radioimmunoassays (RIA), and immunological assays. Study the effect of X-irradiation on MuMTV expression and mammary tumor incidence in Balb/c mice in vivo. Treat preneoplastic nodules derived from hyperplastic alveolar nodules (HAN) in vitro with exogenous factors (i.e., chemicals and hormones) and monitor for transplantability in vivo, MuMTV transcription, and selected clelular growth characteristics.